1. Field of the Invention
The present invention generally relates to a fixing device including an endless belt as a fixing member, and an image forming apparatus such as a facsimile machine, a printer, a copier, a multi-function peripheral and the like including the fixing device and employing an electrophotographic scheme or an electrostatic recording scheme.
2. Description of the Related Art
Conventionally, various image forming apparatuses including a copier, a printer and the like and employing the electrophotographic scheme have been developed and disclosed. In typical image forming processes of the image forming apparatuses, a latent image is formed on a surface of a photosensitive drum as an image carrier; the latent image on the photosensitive drum is developed to be visualized by using toner as developer; the developed image is transferred onto a recording paper (which is also called a sheet or a recording medium) by a transfer device, so that the image (toner image) is carried onto the recording sheet; and the toner image on the recording paper is fixed by being pressed and heated by a fixing device.
In the fixing device, a fixing member is in contact with a pressing member so that a nip section is formed. The fixing member may include a pair of rollers facing each other, or a belt, or a combination thereof. The recording paper is sandwiched in the formed nip section, where heat and pressure are applied to the recording paper so that the toner image is fixed on the recording paper.
As an example of the fixing device, a fixing belt stretched by plural roller members is used as the fixing member (see, for example, Japanese Patent Application Publication No. 11-2982 (Patent Document 1)). The fixing device using such a fixing belt typically include the fixing belt (endless belt) as the fixing member, plural roller members for stretching and supporting the fixing belt, a heater included in one of the roller members, and a pressing roller (as a pressing member). The heater is used to heat the fixing belt via the roller member. The toner image on a recording medium fed to the nip section formed between the fixing belt and the pressing roller is fixed to the recording medium by the heat and the pressure applied at the nip section (belt fixing method).
Further, as another example of the fixing device used in the image forming apparatus, the fixing device includes a fixing member slidably connected with an inner surface of a rotating fixing member. For example, Japanese Patent Application Publication No. 4-44075 (Patent Document 2) discloses the fixing device employing a film heating method, in which a fixing nip section is formed by sandwiching a heat resistant film (fixing film) between a ceramic heater as a heating element and a pressing roller as a pressing member. A recording material (recording medium) on which an unfixed toner image to be fixed is carried is introduced between the film and the pressing roller and is fed along the film while being sandwiched. By doing this, in the nip section, the heat from the ceramic heater is conducted to the recording material via the film. As a result, the unfixed toner image is heat-press fixed to the surface of the recording material due to the heat and pressure applied to the nip section. The fixing device employing the film heating method may serve as an on-demand type device by using the ceramic heater and a member having a lower heat capacity as the film. Further, in the fixing device employing the film heating method, power may be supplied to the ceramic heater to heat and maintain the ceramic heater at a predetermined temperature only when the image forming operation of the image forming apparatus is to be executed. Therefore, the fixing device employing the film heating method has advantages to reduce the wait time period from when the power of the image forming apparatus is turned on to when the image forming apparatus is ready to perform image forming (quick start capability) and to remarkably reduce the energy consumption in a stand-by mode (lower energy consumption).
Further, Japanese Patent Application Publication Nos. 8-292903 (Patent Document 3) and 10-213984 (Patent Document 4) disclose fixing devices employing a pressing belt method, which includes a rotatable heat fixing roller having an elastically deformed surface, an endless belt (pressing belt) which is movable while being in contact with the heat fixing roller, and a non-rotatable pressing pad disposed inside the endless belt. The pressing pad presses the endless belt to the heat fixing roller to form a belt nip section where a recording paper is fed between the endless belt and the heat fixing roller, and elastically deforms the surface of the heat fixing roller. According to this fixing method, the belt is used as the pressing member. Therefore, a contact area between the sheet and the roller can be enlarged. As a result, the heat conductivity efficiency can be remarkably improved, the energy consumption can be reduced, and size of the device can also be reduced.
In the fixing device of Patent Document 1, it may be possible to reduce the operating time when compared with the apparatus using the fixing roller. However, there may be limitations to reduce a warm-up time (which is a time period necessary to increase the temperature to perform printing) and a first printing time (which is a time period from when print request is received to when the apparatus is ready to start printing and actually discharge the printed sheet).
In contrast, in the fixing device of Patent Document 2, due to use of the member having a lower heat capacity, the warm-up time and the fast printing time may be reduced and the size of the device may also be reduce. However, in the fixing device of Patent Document 2, there may be problems in durability and in temperature stability of the belt. Namely, the abrasion resistance in sliding between the ceramic heater as a heat source and the inside surface of the belt may not be sufficient, and continuous abrasion between the ceramic heater and the belt may wear (damage) the surface and may increase the friction resistance between the ceramic heater and the belt, which may cause unstable movement of the belt. Otherwise, for example, the driving torque of the fixing device may increase. As a result, the transfer sheet on which an image is to be formed may slip, which may cause misalignment of the image. Further, the stress applied to the driving gear may increase, which may damage the gear (Problem 1).
Further, in the fixing device employing the film heating method, the belt is locally heated at the nip section. Because of this feature, the temperature of the belt just before the belt enters into the nip section become minimum. Therefore, (especially when the belt is rotated fast,) a fixing failure (e.g., insufficient fixing) is likely to occur (Problem 2).
On the other hand, in Patent Document 3, as a low friction sheet (sheet shaped sliding member), a glass fiber sheet in which PTFE (polytetrafluoroethylene) is impregnated (i.e., PTFE impregnated glass cloth) is provided on the surface of the pressing pad to improve the problem in the slidability between the inside surface of the belt and the fixing member. However, in such a fixing device employing the pressing belt method, the heat capacity of the fixing roller is high. Therefore, it may take more time to increase the temperature. Namely, the warm-up time may become long (Problem 3).
In regard to the Problems 1 to 3, Japanese Patent Application Publication Nos. 2007-334205 (Patent Document 5) and 2008-158482 (Patent Document 6) disclose fixing devices that enable heating the entire fixing belt by providing a facing member (metal heat conductor) having substantially a pipe shape disposed on the inner periphery side of the endless fixing belt and a resistor heater such as a ceramic heater disposed on the inner periphery side of the facing member and heating the facing member. By having this configuration, the warm-up time and the first printing time may be reduced, and further heat shortage in fast rotating may be resolved.
In the fixing devices described in Patent Documents 5 and 6, the metal heat conductor disposed so as to face the inner periphery of the fixing member has a substantially circular shape. Due to the shape, when the heat source is disposed inside the metal heat conductor, the circumferential length of the metal heat conductor may have to be increased. As a result, not only the size of the fixing device but also the heat capacity of the entire fixing device may be increased, thereby increasing the energy consumption. To reduce the higher energy consumption, a sheet heating element having higher heat efficiency than that of a halogen heater may be used. However, to that end, it may become necessary to further provide a pressing mechanism inside the sheet heating element to press the sheet heating element so that the sheet heating element can be in close contact with the fixing member or the metal heat conductor with a uniform surface pressure. Namely, even when the sheet heating element is used instead of using the halogen heater, the pressing mechanism including the sheet heating element may be required to be contained inside the fixing member or the metal heat conductor. As a result, similar to the case described above, the circumferential length of the fixing member may be increased.